Good morning! To those on the Old Calendar, a Merry Christmas to you, and to those on the New, a blessed Theophany (and feast of St. John) to all.
On occasion, the subject of "who can read my PMs" arises in threads, along the lines of, "I'm sure the (insert: mods, administrators, global mods) can go through and read my PMs." This is usually offered as an insult to the mods (insinuating that they'd do anything to try and impugn one's reputation), or as a conspiratorial volley ("they're out to get us!").
The simple answer: for the most part, we cannot. There is no access point in the Simple Machines Forum (SMF) software for us to read anyone's PMs - the SMF developers are opposed to such access in principle (as are we), and thus made no access point. Thus, we don't have a way to snoop on your PMs, and we frankly don't want to. We are perfectly aware that there are (and always have been) users who are dissatisfied with how the forum is run who choose to PM each other to complain, plan, etc. - generally, they or someone they communicate with is open with us about their existence. But even in those cases, we're not interested in reading your PMs. There are other factors (how they're organized, the sheer volume, etc.) that would deter anyone from looking into them anyway.
Two notable exceptions:
1. If someone reports your PM to us, then yes, we can read it - but only what is specifically reported to us, not others (i.e. if someone reports one harassing PM, we're not able to go back and see any history of back-and-forth, unless those were also individually reported to us). This is included as a safe-guard against PM harassment. Easiest way to avoid this: don't harass people (via PM, or any other means).
2. Anastasios, site owner and technical administrator, could theoretically slog through the MySQL database to find PMs. But it wouldn't be easy or convenient. He's the only one with server & database access and has also expressed in the past that he's not interested in snooping on people's PMs.
So rest easy that the moderators, GMs, and Administrators have neither the access nor the inclination to read your PMs.